The document U.S. Pat. No. 6,176,323 describes a drilling system for drilling oilfield boreholes or wellbores utilizing a drill string having a drilling assembly conveyed downhole by a tubing (usually a drill pipe or coiled tubing). The drilling assembly includes a bottom hole assembly (BHA) and a drill bit. The bottom hole assembly preferably contains commonly used measurement-while-drilling sensors. The drill string also contains a variety of sensors for determining downhole various properties of the drilling fluid. Sensors are provided to determine density, viscosity, flow rate, clarity, compressibility, pressure and temperature of the drilling fluid at one or more downhole locations. Chemical detection sensors for detecting the presence of chemical species such as methane (CH4), carbon dioxide (CO2), hydrogen sulfide (H2S), etc. are disposed in the drilling assembly. Sensors for determining fluid density, viscosity, pH, solid content, fluid clarity, fluid compressibility, and a spectroscopy sensor are also disposed in the bottom hole assembly. Data from such sensors is processed downhole and/or at the surface. Corrective actions are taken based upon the downhole measurements at the surface which may require altering the drilling fluid composition, altering the drilling fluid pump rate or shutting down the operation to clean wellbore. The drilling system contains one or more models, which may be stored in memory downhole or at the surface. These models are utilized by the downhole processor and the surface computer to determine desired fluid parameters for continued drilling. The drilling system is dynamic, in that the downhole fluid sensor data is utilized to update models and algorithms during drilling of the wellbore and the updated models are then utilized for continued drilling operations.
Such bottom hole assembly performs optical measurements using optical window.
The bottom hole assembly operates in harsh environment, namely extreme conditions including high pressure from several hundred bars up to 2000 bars, high temperature up to 200° C., presence of corrosive fluids such as sulfuric acid, presence and contamination by solid particles such as scales, asphalthenes, sand particles, as well as multiphasic flow conditions (oil, gas, water), and mud. Further, there are also is the space and power constraints associated to bottom hole assembly deployment. Furthermore, there is the high shocks environment associated to drilling operations.
The drawbacks of the design described in document U.S. Pat. No. 6,176,323 are the following:                Low performance of compositional analysis—The optical path length selection (light travel distance inside the fluid) is constrained by the fluid flow restriction that can be accepted without altering the sampling process and/or the representativeness of the fluid sample present in between the two windows. Distance between windows below 1 mm is in practice very difficult to implement and typical optical path lengths for current commercial tools are within the 2-5 mm range. This limitation has forced engineers to favor optical analysis in the visible—near infrared spectrum (i.e. 400 to 1500 nm wavelengths) where attenuation factors in hydrocarbon fluids are relatively low, despite the fact that absorption peaks are related to overtones of the vibration modes of the molecules to be detected and suffer from peak enlargements and overlaps from different compounds. This leads to complex interpretation and relatively poor robustness and performance of such compositional analysis.        Optical cell contamination—Relatively large optical window surface (several mm2) and even more importantly its flat surface (large curvature ratio superior to 1 mm) favor the formation of droplets of fluids sticking on the window surface and altering analysis.        Complex and expensive hardware—The state of the art window assembly relies on sapphire disk or tube brazed onto the metal housing of the measurement cell) which critical in order to withstand high pressure and high temperature.        Optical path—The optical path length is fixed and can only be changed at the manufacturing step of the cell requiring a complete redesign of the measurement cell.        Cleaning—The complex mechanical configuration of the cell flow line and windows leads to dead volumes which make the cleaning of the cell flow line during the early phase of the sampling a critical step. In practice measurements are affected by fluid contamination and very slow to react.        